Enthalpies of sublimation and fusion for N-acetyl substituted glycine,L-alanine,and D-leucine amides

Enthalpies and entropies of sublimation for N-acetylglycine amide (NAGA), N-acetyl-L-alanine amide (L-NAAA), and N-acetyl-D-leucine amide (D-NALA) were determined from the dependence of their vapour pressures on temperature, as measured by the torsion-effusion method. Enthalpies and temperatures of fusion were measured by differential scanning calorimetry (DSC) and entropies of fusion were derived. No solid-to-solid transitions were detected from r.t. to fusion. Enthalpies of sublimation and fusion were combined to evaluate enthalpies of vaporization of the melts. The experimental results decreased inversely to the molecular mass. An interpretation of this trend in terms of the crystalline structure of these compounds is proposed.